Nuclear fuel assemblies for powering nuclear reactors generally consist of large numbers of fuel rods contained in discrete fuel rod assemblies. These assemblies or cells generally consist of a bottom end fitting or nozzle, a plurality of fuel rods extending upwardly therefrom and spaced from each other in a square or triangular pitch configuration, spacer grids situated periodically along the length of the assembly for support and orientation of the fuel rods, a plurality of control guide tubes interspersed throughout the rod assembly, and a top end fitting or cap. Moreover, the assembly is installed and removed from the reactor as a unit.
When the nuclear fuel rods have expended a large amount of their available energy, the fuel rods are considered to be "spent," and the fuel rod assembly is pulled from the reactor and temporarily stored in an adjacent pool until the assemblies are transported to a reprocessing center or to permanent or temporary storage. Even though the rods are considered "spent," they are still highly radioactive and constitute a very real hazard both to personnel and to property.
In general, there are a number of alternatives available for disposition of the radioactive spent fuel rods, none of which is totally satisfactory. The fuel rod assemblies can be enclosed in a suitable basket and cask arrangement and transported to a storage facility, or possibly, to a reprocessing plant. A second alternative is to store the spent fuel in a dry storage system. Dry storage entails either the use of a large number of metal casks or the building of massive concrete containers either above or below ground, which is a very expensive process, and, where the storage system is above ground, it is often not acceptable to people living or working in its vicinity. A third alternative is the storage of the fuel units in the existing water pool originally designed for temporary storage. This type of storage is the simplest and cheapest, since the fuel rod assemblies can remain in the pool and be left there until the appropriate governmental agency or other agency collects them, often at the end of the life of the nuclear plant. However, such storage pools have a limited capacity, and, where they are adjacent to the nuclear reactor, when one becomes full the construction of a new pool is necessary.
Numerous attempts have been made to increase the capacity of a pool through a process known as fuel rod compaction or consolidation. This process, in brief, comprises removing the fuel rods from each fuel rod assembly and placing them in a storage canister where they are placed in rows with minimal spacing, most often in a square or triangular array. It is possible, with this process, to place the fuel rods from two or more fuel assemblies into a single storage canister, thereby achieving approximately a 1:2 reduction in required pool volume, or, conversely, a 2:1 increase in pool storage capacity. Examples of such rod consolidation systems are shown in U.S. Pat. No. 4,659,536 of Baudro; U.S. Pat. No. 4,731,219 of Beneck, et al.; and U.S. Pat. No. 5,000,906 of Ellingson, et al. However, successful consolidation has been an elusive goal for a number of reasons. Inasmuch as the pools are approximately forty feet deep, and inasmuch as the rods must remain immersed in the water at all times, all of the consolidation operations must be performed under the shielding and cooling water. In addition, even though the rods are kept under water, the process could be quite hazardous to personnel performing the operation. The rods themselves, because of their dimensions, for example, one-half inch in diameter and eighteen feet long, are subject to flexing and bending. As a consequence, placing the rods in a desired location can be quite difficult.
Prior art arrangements, such as the Beneck et al. system, for achieving rod consolidation have included a system whereby the rods are pulled out row-by-row, as in, for example, a 14.times.14 matrix of rods, lifted and deposited in a tapered interim storage container, which tapers from a large area top opening to a bottom that has the area of a storage canister. After the intermediate container has the rods from approximately two fuel assemblies deposited therein, the intermediate container is placed over a storage canister, and the bottom plate of the tapered container is lowered or removed to cause the rods to slide into the storage canister. If the rods jam or stick, as they often do, they must be pushed from above the pool by operators using long rods. This last operation is made more difficult in that the rods develop on their outside surfaces what is referred to in the trade as "crud". When the fuel rods are pulled, this radioactive crud is scraped off and clouds the water making it difficult for the operators to see what they are doing and contaminating the pool. The method just described has proven to be quite slow and complicated, and can be hazardous to personnel.
After a storage canister has been filled, it is generally capped by a lid member. In accordance with U.S. government requirements for storing spent nuclear fuel rods, the lid must be lockable or self-locking with tamper proof, or tamper indicating mechanisms. Meticulous records must be kept by the operator as to just what has been put in the canister and when, and the lock and tamper indicator is intended to insure that what has been put in the canister remains there, undisturbed. An example of a storage canister and a lid locking mechanism is disclosed in U.S. Pat. No. 4,474,727 of Kmonk, et al.